Is it actually possible to create multipage TIFF files in the program? I go to Tools>Multipage File>Create, add a number of files and then select 'TIFF revision 6' but it is only the first file in my list that appears in the newly created TIFF file, no sign of multiple pages at all. Am I missing something? Multiple-page PDFs are created without problem though.

Creating from single-page TIFF files?

No problem for me with XnView Classic 2.43.

Are you using the small arrows on the toolbar to step through the pages?